Compound semiconductor substrate

ABSTRACT

A compound semiconductor substrate which inhibits the generation of a crack or a warp and is preferable for a normally-off type high breakdown voltage device, arranged that a multilayer buffer layer  2  in which Al x Ga 1-x N single crystal layers (0.6≦X≦1.0)  21  containing carbon from 1×10 18  atoms/cm 3  to 1×10 21  atoms/cm 3  and Al y Ga 1-y N single crystal layers (0.1≦y≦0.5)  22  containing carbon from 1×10 17  atoms/cm 3  to 1×10 21  atoms/cm 3  are alternately and repeatedly stacked in order, and a nitride active layer  3  provided with an electron transport layer  31  having a carbon concentration of 5×10 17  atoms/cm 3  or less and an electron supply layer  32  are deposited on a Si single crystal substrate  1  in order. The carbon concentrations of the Al x Ga 1-x N single crystal layers  21  and that of the Al y Ga 1-y N single crystal layers  22  respectively decrease from the substrate  1  side towards the above-mentioned active layer  3  side. In this way, the compound semiconductor substrate is produced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a compound semiconductor substrate usedpreferably for a high speed or high breakdown voltage semiconductordevice etc.

2. Description of the Related Art

A semiconductor device using a nitride semiconductor represented bygallium nitride in hexagonal crystal form (GaN), aluminum nitride inhexagonal crystal form (AlN), etc., allows a high breakdown voltage anda high frequency, surpasses properties of a silicon (Si) semiconductordevice which is currently dominant, and is expected to be used for powerdevices etc.

As a method of fabricating the semiconductor device of this type and thecompound semiconductor substrate used for its fabrication at low cost,it is known that a buffer area constituted by a plurality of AlGaN-basednitride semiconductor layers whose compositions are different is formedon a silicon (Si) or silicon carbide (SiC) single crystal substrate, anda nitride semiconductor active layer is formed on the layers of thebuffer area.

For example, Japanese Patent Application Publication No. 2007-67077(patent document 1) discloses that a plurality of first buffer layerscomposed of Al_(x)Ga_(1-x)N (0.5≦x≦1) and a plurality of second bufferlayers composed of Al_(y)Ga_(1-y)N (0.01≦y≦0.2) are alternately stacked.

Further, Japanese Patent Application Publication No. 2008-171843 (patentdocument 2) discloses that a buffer layer is formed having a compositelayer where a second layer formed using a nitride-based compoundsemiconductor in which an Al content is 0.8 or more is stacked on afirst layer formed using a nitride-based compound semiconductor in whichan Al content is 0.2 or less.

According to the buffer layer with the structure as disclosed in patentdocument 1 above, it is effective to form a nitride semiconductor whichis flat and smooth without a crack. However, its breakdown voltage perunit film thickness is low and the compound semiconductor substratesuitable for a high breakdown voltage device may not be obtained.Further, it is not suitable for achieving a normally-off state.

On the other hand, patent document 2 discloses that it is possible toattain a high breakdown voltage by setting a carbon concentration of thefirst layer of the above-mentioned buffer layer as 1×10¹⁷ atoms/cm³ to1×10²⁰ atoms/cm³. However, even if the device is provided with thebuffer layer having such a structure, it does not fully meet the recentdemands for the high breakdown voltage and achieving the normally-offstate.

SUMMARY OF THE INVENTION

The present invention is made to solve the above-mentioned technicalproblems, and aims to provide at low cost a compound semiconductorsubstrate which inhibits the generation of a crack or a warp and issuitable for a normally-off type high breakdown voltage device.

The compound semiconductor substrate in accordance with the presentinvention is arranged such that a multilayer buffer layer in whichAl_(x)Ga_(1-X)N single crystal layers (0.6≦X≦1.0) containing carbon from1×10¹⁸ atoms/cm³ to 1×10²¹ atoms/cm³ and Al_(y)Ga_(1-y)N single crystallayers (0≦y≦0.5) containing carbon from 1×10¹⁷ atoms/cm³ to 1×10²¹atoms/cm³ are alternately and repeatedly stacked in order, and a nitrideactive layer comprising an electron transport layer having a carbonconcentration of 5×10¹⁷ atoms/cm³ or less and an electron supply layerare deposited on a Si single crystal substrate in order, andcharacterized in that the carbon concentrations of the above-mentionedAl_(x)Ga_(1-x)N single crystal layers and the carbon concentration ofthe Al_(y)Ga_(1-y)N single crystal layers respectively decrease from theabove-mentioned substrate side towards the above-mentioned active layerside.

By providing such a multilayer buffer layer, it is possible to inhibitthe generation of the crack in the nitride active layer and the warp ofthe substrate and attain the normally-off state and the high breakdownvoltage of the device using the substrate.

In the above-mentioned compound semiconductor substrate, it ispreferable that the above-mentioned Al_(y)Ga_(1-y)N single crystal layeris of 0.1≦y≦0.5 in terms of allowing the high breakdown voltage and thenormally-off state.

Further, it is preferable that the carbon concentration of theabove-mentioned Al_(x)Ga_(1-x)N single crystal layer is higher than thecarbon concentration of the Al_(y)Ga_(1-y)N single crystal layerimmediately above the former.

A lattice constant difference between the Al_(x)Ga_(1-x)N single crystallayer having a high carbon concentration and the crystal of theAl_(y)Ga_(1-y)N single crystal layer immediately above the formerbecomes large, which may produce strong compressive stress.

Further, it is preferable that the above-mentioned multilayer bufferlayer contains boron from 5×10¹⁶ atoms/cm³ to 1×10¹⁹ atoms/cm³.

Containing boron at the above-mentioned concentration range, the carbonconcentrations of the above-mentioned Al_(x)Ga_(1-x)N single crystallayer and the above-mentioned Al_(y)Ga_(1-y)N single crystal layerincrease, thus achieving the improvement in breakdown voltage.

Furthermore, it is preferable that the above-mentioned electrontransport layer is an Al_(z)Ga_(1-z)N single crystal layer (0≦z≦0.01).

In terms of improving the high speed of the device, it is preferablethat an Al concentration in the above-mentioned electron transport layeris as low as possible.

Further, it is preferable that the above-mentioned electron transportlayer of the above-mentioned compound semiconductor substrate has athickness of from 1 nm to 500 nm and it can be used preferably for anormally-off type switching device.

According to the present invention, it is possible to provide at lowcost the compound semiconductor substrate which inhibits the crack ofthe nitride active layer from generating and avoids the warp caused bythick film formation of the nitride semiconductor.

Furthermore, the compound semiconductor substrate in accordance with thepresent invention can suitably be applied to the high breakdown voltagedevice, especially the normally-off type switching device, since it ispossible to improve the breakdown voltage by producing the device usingthe substrate.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic sectional view illustrating a layer structure of acompound semiconductor substrate in accordance with the presentinvention.

FIG. 2 is a table showing evaluation results of Samples 1-8.

FIG. 3 is a table showing evaluation results of Samples 9-16.

FIG. 4 is a table showing evaluation results of Samples 17-20.

FIG. 5 is a table showing evaluation results of Samples 21-27.

FIG. 6 is a graph showing a profile for carbon concentrations againstfilm thicknesses, from a Si single crystal substrate surface, of amultilayer buffer layer and a nitride active layer of the compoundsemiconductor substrate according to Samples 28-32.

FIG. 7 is a table showing evaluation results of Samples 28-32.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the present invention will be described in detail withreference to the drawings.

FIG. 1 schematically illustrates a layer structure of a compoundsemiconductor substrate in accordance with the present invention.

The compound semiconductor substrate illustrated in FIG. 1 has thestructure in which a multilayer buffer layer 2 and a nitride activelayer 3 which is constituted by an electron transport layer 31 and anelectron supply layer 32 are stacked on a Si single crystal substrate 1in order.

The above-mentioned multilayer buffer layer 2 is of a multilayerstructure where Al_(x)Ga_(1-x)N single crystal layers (0.0) 21 andAl_(y)Ga_(1-y)N single crystal layers (0≦y≦0.5) 22 are alternately andrepeatedly stacked in this order, from the Si substrate 1 side.

Further, the above-mentioned Al_(x)Ga_(1-x)N single crystal layer(0.6≦x≦1.0) 21 contains carbon from 1×10¹⁸ atoms/cm³ to 1×10²¹atoms/cm³, and the above-mentioned Al_(y)Ga_(1-y)N single crystal layer(0≦y≦0.5) 22 contains carbon from 1×10¹⁷ atoms/cm³ to 1×10²¹ atoms/cm³.

By providing such a multilayer buffer layer 2, it is possible to inhibita crack from generating in the nitride active layer 3 and the wholesubstrate from warping. In addition, an effect that a device using thissubstrate can be produced to withstand a high voltage is obtained.

In the case where a multilayer buffer layer is formed in such a way thata plurality of conventional AlGaN-based single crystals with differentcompositions are alternately stacked, carriers generated due toproperties different between a Si single crystal substrate and eachsemiconductor film remain in the multilayer buffer layer, therebyinhibiting the device from withstanding a high-voltage.

In order to control such a carrier, it is advantageous that themultilayer buffer layer contains carbon therein.

In the present invention, the breakdown voltage of the device can beraised by intentionally adding carbon so as to provide the concentrationrange as mentioned above.

In the above-mentioned Al_(x)Ga_(1-x)N single crystal layer 21, the Alcontent x is set as 0.6≦x≦1.0. Further, in the above-mentionedAl_(y)Ga_(1-y)N single crystal layer 22, the Al content y is set as0≦y≦0.5, preferably 0.1≦y≦0.5.

In the case where x<0.6 and y>0.5, compressive stress unlikely to occursince a difference between a crystal-lattice constant of theAl_(x)Ga_(1-x)N single crystal layer and that of the Al_(y)Ga_(1-y)Nsingle crystal layer is small.

On the other hand, in the case where y<0.1, the Al content of theAl_(y)Ga_(1-y)N layer is too low. Therefore, only a low breakdownvoltage device as conventional or a conventional normally-on type deviceis obtained.

As described above, in the compound semiconductor substrate inaccordance with the present invention, although the multilayer bufferlayer 2 is caused to contain carbon at a predetermined concentration, itis preferable that carbon concentrations incline so that both the carbonconcentration of the above-mentioned Al_(x)Ga_(1-x)N single crystallayer 21 and the carbon concentration of the Al_(y)Ga_(1-y)N singlecrystal layer 22 respectively decrease from the above-mentionedsubstrate 1 side towards the above-mentioned active-layer 3 side.

Since the carbon concentration is higher on the substrate 1 side,residual carriers decrease, thus increasing the breakdown voltage.Physically, Fermi level is away from a conduction band.

Further, when causing the closest side of active-layer 3 to be at a lowcarbon concentration, in the case where a gate voltage is not applied,the carrier is not excited.

Physically, the Fermi level approaches the conduction band, and apositive voltage (several V) at a gate allows on/off control in thedevice, i.e. leading to a normally-off state.

Thus, in addition to the increased breakdown voltage, the normally-offstate is attained by the above-mentioned inclination of the carbonconcentrations in the Al_(x)Ga_(1-x)N single crystal layer 21 and theAl_(y)Ga_(1-y)N single crystal layer 22.

Therefore, in the arrangement where the carbon concentration simplydecreases at a predetermined rate as a whole from the substrate 1 sidetowards the active-layer 3 side, the normally-off state is not attained.The above-mentioned carbon concentration change and the layer structurein accordance with the present invention are important to realize a goodnormally-off type and high breakdown voltage device.

Further, let the above-mentioned Al_(x)Ga_(1-x)N single crystal layer 21and the Al_(y)Ga_(1-y)N single crystal layer 22 immediately above theformer be one pair, it is preferable that the Al_(x)Ga_(1-x)N singlecrystal layer 21 (lower layer) has a higher carbon concentration thanthat of the Al_(y)Ga_(1-y)N single crystal layer 22 (upper layer).

Since the Al_(x)Ga_(1-x)N single crystal layer 21 with the higher carbonconcentration is of a soft crystal, it is likely to generate dislocationand relax the crystal. Since the relaxed crystal has a large latticeconstant difference from that of the crystal of the Al_(y)Ga_(1-y)Nsingle crystal layer 22 immediately above the former, strong compressivestress takes place.

Further, it is preferable that the above-mentioned multilayer bufferlayer 2 contains boron from 5×10¹⁶ atoms/cm³ to 1×10¹⁹ atoms/cm³.

Containing boron at the above-mentioned concentration range, the carbonconcentrations of the Al_(x)Ga_(1-x)N single crystal layer 21 and theAl_(y)Ga_(1-y)N single crystal layer 22 can be increased further, thusachieving the improvement in breakdown voltage.

It is assumed that, in crystal lattices of the Al_(x)Ga_(1-x)N singlecrystal layer 21 and the Al_(y)Ga_(1-y)N single crystal layer 22, boronenters lattice positions of Ga, so that the lattice constants increaseand more carbons are taken into the interstitial site.

In the case where the above-mentioned boron concentration is less than5×10¹⁶ atoms/cm³, the carbon concentration of the multilayer bufferlayer decreases, and the breakdown voltage of the device becomes as lowas the conventional one.

On the other hand, in the case where the above-mentioned boronconcentration exceeds 1×10¹⁹ atoms/cm³, the carbon concentration exceedsthe maximum value as specified above, and crystallinity is worsened.

Further, in the above-mentioned multilayer buffer layer 2, it ispreferable that the Al_(x)Ga_(1-x)N single crystal layer 21 has athickness of from 1 nm to 50 nm and the Al_(y)Ga_(1-y)N single crystallayer 22 has a thickness of from 10 nm to 500 nm.

The two types of AlGaN-based single crystal layers which constitute themultilayer buffer layer 2 are formed to have different film thicknessesas described above, so that controllability of the stress generatedbetween the substrate and the multilayer buffer layer made of thenitride semiconductor can be improved. For example, warp can be reducedto 30 μm or less in the substrate having a diameter of 4 inches.

In the case where the thickness of the above-mentioned Al_(x)Ga_(1-x)Nsingle crystal layer 21 is less than 1 nm or in the case where thethickness of the Al_(y)Ga_(1-y)N single crystal layer 22 is less than 10nm, crystallinity required for the buffer is not obtained.

On the other hand, in the case where the thickness of theabove-mentioned Al_(x)Ga_(1-x)N single crystal layer 21 exceeds 50 nm orin the case where the thickness of the Al_(y)Ga_(1-y)N single crystallayer 22 exceeds 500 nm, manufacture costs increase and a reasonableefficiency may not be obtained.

Further, in the case where the Al_(x)Ga_(1-x)N single crystal layer 21and the Al_(y)Ga_(1-y)N single crystal layer 22 immediately above theformer are set as one pair, it is preferable that 5-100 pairs thereofare repeatedly stacked for the above-mentioned multilayer buffer layer2.

Too small a number of the stacked pairs result in insufficient stressrelaxation by the multilayer buffer layer, the effect of inhibiting thecrack or warp is not fully obtained.

On the other hand, in the case where there are too many stacked pairs,it becomes costly and is inferior in respect of manufacturingefficiency, which is not preferred.

Further, it is preferable that the carbon concentration of the electrontransport layer 31 formed on the above-mentioned multilayer buffer layer2 is 5×10¹⁷ atoms/cm³ or less.

The presence of carbon in the electron transport layer 31 scatterselectrons and the mobility is reduced, so that the rapidity of thedevice decreases. Thus, it is preferable that the carbon concentrationis lower. When the above-mentioned carbon concentration is 5×10¹⁷atoms/cm³ or less, a difference in rapidity of the device is small.

In addition, according to the measurement by a secondary ion massspectrometry (SIMS) process, the minimum limit of detection of thecarbon concentration is 5×10¹⁶ atoms/cm³ in the current technical level.In the case of exceeding 5×10¹⁷ atoms/cm³, it was confirmed that therapidity of the device decreased significantly.

It is preferable that the above-mentioned electron transport layer 31 isan Al_(z)Ga_(1-z)N single crystal layer (0≦z≦0.01).

The presence of aluminum (Al) in the electron transport layer 31scatters electrons and the mobility is reduced, so that the rapidity ofthe device also decreases in this case. Therefore, the lower the Alconcentration, the more preferable. However, it is practically difficultto let z=0 in terms of contamination of the residual Al in a furnacewhen forming the electron transport layer 31. Thus, it is preferablethat z≦0.01 as a concentration at which Al is contained intentionally.

More preferably, z=0 i.e., the above-mentioned electron transport layer31 is a GaN layer. Thus, the electron transport layer 31 is of the GaNlayer and differentiated from the above-mentioned multilayer bufferlayer 2 in terms of the Al concentration. This means physically thatFermi level is away from the conduction band, which is preferred also interms of the normally-off state.

Further, it is preferable that the above-mentioned electron transportlayer 31 is formed to have a thickness of approximately 1-5000 nm inorder to avoid the generation of a crack.

The above-described compound semiconductor substrate in accordance withthe present invention can be used preferably for a normally-off typeswitching device by forming the above-mentioned electron transport layer31 to have a thickness of from 1 nm to 500 nm.

By “normally-off type device” we mean one which is in a stop (OFF) statein the case where a voltage is not applied to a control electrodereferred to as a gate electrode (zero voltage) and which is in anoperation (ON) state when a positive voltage is applied.

On the other hand, by “normally-on type device” we mean one which is inthe operation (ON) state without applying a voltage to the gateelectrode (control electrode) (zero voltage) and which is in the stop(OFF) state when a negative voltage is applied.

In terms of safety of a circuit etc., the switching device of thenormally-off type is preferred to that of the normally-on type and thecompound semiconductor substrate in accordance with the presentinvention is very useful for forming the device.

The process of manufacturing the above-mentioned Si single crystalsubstrate 1 is not particularly limited. It may be manufactured by theCzochralski (CZ) process, or may be manufactured by a floating zone (FZ)process. Further, it may be manufactured in such a way that a Si singlecrystal layer is epitaxially grown on the Si single crystal substrate bya vapor deposition process (Si epitaxial substrate).

By epitaxially growing the nitride semiconductor on the Si singlecrystal substrate, it is possible to utilize the apparatuses andtechniques which are used in the conventional Si semiconductormanufacturing process, whereby a large diameter substrate can bemanufactured at low cost.

In addition, before forming the multilayer buffer layer 2 on theabove-mentioned Si single crystal substrate 1, it is possible to form anAlN single crystal layer etc. as an initial buffer in order to improveaffinity of Si single crystal substrate 1 to the Al_(x)Ga_(1-x)N singlecrystal layer 21.

Since Ga very highly reacts with Si, in the case where Ga adheres to aSi substrate surface in initial stages of growth, roughness of the Sisingle crystal substrate surface arises as a result of a melt backetching reason.

For this reason, the above-mentioned initial buffer is effective asprotection and a primer of the Si single crystal substrate surfacebefore forming a multilayer buffer layer.

Further, another layer, such as a modulation doped layer, a spacerlayer, etc., may be formed between the above-mentioned electrontransport layer 31 and the electron supply layer 32. Furthermore,according to an object and use when fabricating the device, it ispossible to form another layer, such as a cap layer, a passivationlayer, etc., on the nitride active layer 3.

In addition, usually, each of the compound semiconductor layers inaccordance with the present invention is deposited and formed byepitaxial growth, but not particularly limited thereto and a processgenerally used may be employed. For example, it is possible to use CVDprocesses including MOCVD (Metal Organic Chemical Vapor Deposition) andPECVD (Plasma Enhanced Chemical Vapor Deposition), a depositing processusing a laser beam, a sputtering process using atmosphere gas, MBE(Molecular Beam Epitaxy) using a molecular beam under high vacuum, MOMBE(Metal Organic Molecular Beam Epitaxy) which is a combination of MOCVDand MBE, etc.

Further, materials used when epitaxially growing each layer are notlimited to those used in the following Example. Source gases with whichcarbon is included may be, for example, acetylene, ethane, propane,trimethyl aluminum, and trimethyl gallium, other than methane.

Furthermore, forming or processing the electrodes when fabricating thecompound semiconductor device by means of the above-described compoundsemiconductor substrate in accordance with the present invention is notparticularly limited, and can be carried out by a general method. Forexample, it is possible to form an electrode at a surface and the backof the above-mentioned substrate with a known material by vacuumdeposition or lithography.

EXAMPLE

Hereinafter, the present invention will be described more particularlywith reference to Example, but the present invention is not limited tothe following Example.

Samples of a compound semiconductor substrate were produced under thefollowing conditions. Each substrate and a device produced using thesubstrate were evaluated as follows:

(Evaluation of Substrate)

As for each compound semiconductor substrate sample, a dislocationdensity in an Al_(z)Ga_(1-z)N single crystal layer used as an electrontransport layer 31 was evaluated by a transmission electron microscope.Further, the generation of warp and a crack was also evaluated with alaser displacement meter and an optical microscope.

(Evaluation of Device)

For each compound semiconductor substrate sample, a recess between arecess gate region and an element isolating region was formed by dryetching. A gold (Au) electrode as a gate electrode and aluminum (Al)electrodes as source and drain electrodes were formed on an active-layerside, and an Al electrode as a back electrode was formed on the backside of a Si substrate, each being carried out by vacuum deposition tomake a compound semiconductor device.

As for the thus obtained compound semiconductor device, while applying avoltage across source and drain electrodes by means of a curve tracer,current-voltage properties at the time of applying a voltage to the gateelectrode were measured to find a pinch off voltage, and a degree of thenormally-off state was evaluated.

Then, while applying a pinch off voltage to the gate electrode,current-voltage properties at the time of applying a voltage across thesource and drain electrodes were measured, to find breakdown voltageproperties.

In addition, the source electrode and the back electrode wereelectrically short-circuited at the time of measuring the device.

Further, an Al electrode as ohmic contact was formed on each compoundsemiconductor substrate sample by vacuum deposition and Hall effectmeasurement was carried out, to thereby find electron mobility in theelectron transport layer and evaluate the rapidity of the device.

Hereinafter, a production process for each compound semiconductorsubstrate sample and conditions are shown.

[Sample 1] (Standard Sample)

A compound semiconductor substrate provided with layer structure asshown in FIG. 1 was produced according to the following processes.

Firstly, a Si single crystal substrate 1 having a diameter of 4 incheswas placed in a MOCVD equipment. Trimethyl aluminum (TMA) gas, NH₃ gas,methane gas, and diborane gas were used as source gases, and an AlNsingle crystal layer (Al_(x)Ga_(1-x)N single crystal layer (x=1)) 21having a thickness of 20 nm and containing carbon at 1×10²⁰ atoms/cm³was formed by vapor phase epitaxy at 1000° C. Further, trimethyl gallium(TMG) gas, TMA gas, NH₃ gas, methane gas, diborane gas were used assource gases, and a GaN single crystal layer (Al_(y)Ga_(1-y)N singlecrystal layer (y=0.2)) 22 having a thickness of 80 nm and containingcarbon at 5×10¹⁹ atoms/cm³ was stacked on the AlN single crystal layerby vapor phase epitaxy at 1000° C. Similarly, these steps werealternately repeated so that carbon concentrations might decrease fromthe above-mentioned substrate side towards the above-mentionedactive-layer side, respectively. Ten layers for each (20 layers intotal) were stacked to form a multilayer buffer layer 2.

TMG gas, TMA gas, NH₃ gas, and methane gas were used as source gases,and an Al_(0.02)Ga_(0.98)N single crystal layer (Al_(z)Ga_(1-z)N layer(z=0.02)) having a thickness of 1000 nm and containing carbon at 5×10¹⁷atoms/cm³ as the electron transport layer 31 was stacked on theabove-mentioned multilayer buffer layer 2 by vapor phase epitaxy at1000° C. Further, the electron supply layer 32 of an Al_(0.25)Ga_(0.75)Nsingle crystal having a thickness of 30 nm was stacked, a nitride activelayer 3 was formed, and the compound semiconductor substrate wasobtained.

In addition, adjustment of the thickness of each layer formed by thevapor phase epitaxy was carried out by adjustment of a gas flow rate anda supply period.

[Samples 2-4] (Compositions of Al_(x)Ga_(1-x)N Single Crystal Layers 21)

In Sample 1, an Al_(z)Ga_(1-z)N layer which was the electron transportlayer 31 was formed into a GaN layer (z=0) having a thickness of 500 nm,and an Al_(y)Ga_(1-y)N single crystal layer 22 was formed into anAl_(0.2)Ga_(0.8)N single crystal layer (y=0.2). The value x of theAl_(x)Ga_(1-x)N single crystal layer 21 was varied to Al_(0.5)Ga_(0.5)N(x=0.5) (Sample 2), Al_(0.6)Ga_(0.4)N (x=0.6) (Sample 3), and AlN (x=1)(Sample 4). The other processes were similar to those for Sample 1, toproduce the compound semiconductor substrates.

[Samples 5-8] (Compositions of Al_(y)Ga_(1-y)N Single Crystal Layers 22)

In Sample 4, the value y of the Al_(y)Ga_(1-y)N single crystal layer 22was varied to GaN (y=0) (Sample 5), Al_(0.1)Ga_(0.9)N (y=0.1) (Sample6), Al_(0.5)Ga_(0.5)N (y=0.5) (Sample 7), and Al_(0.6)Ga_(0.4)N (y=0.6)(Sample 8). The other processes were similar to those for Sample 1, toproduce the compound semiconductor substrates.

Evaluation results of Samples 1-8 are collectively shown in Table 1(FIG. 2).

In addition, Sample 1 is considered as the standard sample whencomparing other Samples with it. Leak current is expressed with arelative index when assuming Sample 1 as 1 in Table 1. The same appliesto the following evaluation results.

As can be seen from the evaluation results shown in Table 1, in the casewhere x<0.6 (Sample 2) and y>0.5 (Sample 8), the warp of the substratewas large, the generation of a crack was also observed, and the leakcurrent of the device was larger.

It was confirmed that Samples except Samples 2 and 8 had goodnormally-off properties and improved the rapidity of the devices.

[Samples 9-11] (carbon concentrations of Al_(x)Ga_(1-x)N single crystallayers 21)

In Sample 4, the carbon concentration of the Al_(x)Ga_(1-x)N singlecrystal layer 21 was varied as shown in Samples 9-11 of the followingTable 2. The other processes were similar to those for Sample 1, toproduce the compound semiconductor substrates.

[Samples 12-15] (Carbon Concentrations of Al_(y)Ga_(1-y)N Single CrystalLayers 22)

In Sample 4, the carbon concentration of the Al_(y)Ga_(1-y)N singlecrystal layer 22 was varied as shown in Samples 12-15 of the followingTable 2. The other processes were similar to those for Sample 1, toproduce the compound semiconductor substrates.

[Sample 16] (Inclination of Carbon Concentration)

After stacking the AlN single crystal layer 21 and the Al_(0.2)Ga_(0.8)Nsingle crystal layer 22 on the Si single crystal substrate 1, therespective carbon concentrations were alternately and repeatedlyincreased, from the above-mentioned substrate side towards theabove-mentioned active layer side, at a predetermined rate to 3×10²⁰atoms/cm³ of the 10th AlN single crystal layer 21 and to 2×10²⁰atoms/cm³ of the 10th AlN_(0.2)Ga_(0.8)N single crystal layer 22, and 10layers for each (20 layers in total) were stacked to form the multilayerbuffer layer 2. The other processes were similar to those for Sample 1,to produce the compound semiconductor substrates.

Evaluation results of Samples 9-16 are collectively shown in Table 2(FIG. 3).

As can be seen from the evaluation results shown in Table 2, in the casewhere the carbon concentration of the Al_(x)Ga_(1-x)N single crystallayer 21 was less than 1×10¹⁸ atoms/cm³ (Sample 9) and the carbonconcentration of the Al_(y)Ga_(1-y)N single crystal layer 22 was lessthan 1×10¹⁷ atoms/cm³ (Sample 12), the breakdown voltages were less than650V, which was not preferred for the device.

Further, in the case where the carbon concentration of theAl_(x)Ga_(1-x)N single crystal layer 21 exceeded 1×10²¹ atoms/cm³(Sample 11) and the carbon concentration of the Al_(y)Ga_(1-y)N singlecrystal layer 22 exceeded 1×10²¹ atoms/cm³ (Sample 15), dislocationdensities of the substrates were equal to or greater than 1×10¹⁰ cm⁻²,which was of inferior crystallinity and not preferred for the device;the leak current of the device was also large.

Further, in the case where the Al_(x)Ga_(1-x)N single crystal layers 21and the Al_(y)Ga_(1-y)N single crystal layers 22 were stacked so thatthe carbon concentrations were inclined so as to increase (Sample 16),the dislocation density of the substrate was high, the warp was alsolarge, and the crack occurred.

It was confirmed that Samples 10 and 16 had good stress-controllingproperties and good normally-off properties, and the rapidity of thedevices was improved.

[Samples 17-20] (Boron Addition (1))

In Sample 4, boron was added into the Al_(x)Ga_(1-x)N single crystallayer 21 and the Al_(y)Ga_(1-y)N single crystal layer 22. On the basisof Sample 4, boron concentrations were varied as shown in Samples 17-20of the following Table 3. The other processes were similar to those forSample 1, to produce the compound semiconductor substrates.

Evaluation results of Samples 17-20 are collectively shown in Table 3(FIG. 4).

As can be seen from the evaluation results shown in Table 3, in the casewhere the added boron concentrations of the Al_(x)Ga_(1-x)N singlecrystal layer 21 and the Al_(y)Ga_(1-y)N single crystal layer 22 werebetween 5×10¹⁶ atoms/cm³ and 1×10¹⁹ atoms/cm³ (Samples 17-19), it wasconfirmed that normally-off properties were good and the rapidity of thedevices was improved.

[Samples 21-27] (Boron Addition (2))

In Sample 18, each parameter of the Al_(x)Ga_(1-x)N single crystal layer21, the Al_(y)Ga_(1-y)N single crystal layer 22, and the Al_(z)Ga_(1-z)Nlayer which was the electron transport layer 31 was varied as shown inSamples 21-27 of the following Table 4. The other processes were similarto those for Sample 1, to produce the compound semiconductor substrates.

Evaluation results of Samples 21-27 are collectively shown in Table 4(FIG. 5).

As can be seen from the evaluation results shown in Table 4, even in thecase where boron was added into the Al_(x)Ga_(1-x)N single crystal layer21 and the Al_(y)Ga_(1-y)N single crystal layer 22 and when eachcomposition and the carbon concentration were within a range specifiedby the present invention, it was confirmed that the stress-controllingproperties and the normally-off properties were good and the rapidity ofthe devices was improved.

[Sample 28] (Inclination of Carbon Concentration when Adding Boron)

In Sample 18, the AlN single crystal layers 21 and the Al_(0.2)Ga_(0.8)Nsingle crystal layers 22 were repeatedly stacked up to seven layers foreach (14 layers in total) on the Si single crystal substrate 1. Then,the carbon concentration of the eighth AlN single crystal layer 21 wasset as 9×10¹⁹ atoms/cm³ and the carbon concentration of the eighthAlN_(0.2)Ga_(0.8)N single crystal layer 22 was set as 4×10¹⁹ atoms/cm³,so that the carbon concentrations might slightly decrease from theabove-mentioned substrate side towards the above-mentioned active-layerside. The stacking was alternately repeated up to the ninth layer foreach. Further, the carbon concentration of the layer the 10th AlN singlecrystal layer 21 was set as 6×10¹⁹ atoms/cm³ and the carbonconcentration of the 10th AlN_(0.2)Ga_(0.8)N single crystal layer 22 wasset as 2×10¹⁹ atoms/cm³, varying a rate of decrease in carbonconcentration. The respective layers were stacked up to 10 layers (20layers in total) to form the multilayer buffer layer 2. The otherprocesses were similar to those for Sample 1, to produce the compoundsemiconductor substrates.

[Samples 29-31] (Inclination of Carbon Concentration when Adding Boron)

In Sample 28, 10 layers for each (20 layers in total) were alternatelyand repeatedly stacked to form the multilayer buffer layer 2 in such away that the carbon concentration of the 10th AlN single crystal layer21 and the AlN_(0.2)Ga_(0.8)N single crystal layers 22 had the values asshown in Samples 29-31 of the following Table 5 and both were decreasedat a predetermined rate from the above-mentioned substrate side towardsthe above-mentioned active-layer side. The other processes were similarto those for Sample 1, to produce the compound semiconductor substrates.

[Sample 32] (Inclination of Carbon Concentration when Adding Boron)

In Sample 28, 10 layers for each (20 layers in total) were alternatelyand repeatedly stacked to form the multilayer buffer layer 2 in such away that the carbon concentration of the 10th AlN single crystal layer21 was 3×10²⁰ atoms/cm³, the carbon concentration of the 10thAlN_(0.2)Ga_(0.8)N single crystal layer 22 was 2×10²⁰ atoms/cm³, andboth were increased from the above-mentioned substrate side towards theabove-mentioned active-layer side. The other processes were similar tothose for Sample 1, to produce the compound semiconductor substrates.

FIG. 6 shows in graph a profile for the carbon concentrations againstfilm thicknesses, from the Si single crystal substrate surface, of themulti layer buffer layers and nitride active layers of the compoundsemiconductor substrates according to samples 28-32.

Further, the evaluation results of Samples 28-32 are collectively shownin Table 5 (FIG. 7).

As can be seen from the evaluation results shown in Table 5, even in thecase where boron was added into the Al_(x)Ga_(1-x)N single crystal layer21 and the Al_(y)Ga_(1-y)N single crystal layer 22, when the multilayerbuffer layers were formed so that each carbon concentration mightdecrease within a predetermined range (Samples 28-30), it was confirmedthat the stress-controlling properties and the normally-off propertieswere good and the rapidity of the devices was improved.

1. A compound semiconductor substrate, arranged such that a multilayerbuffer layer in which Al_(x)Ga_(1-x)N single crystal layers (0.6≦X≦1.0)containing carbon from 1×10¹⁸ atoms/cm³ to 1×10²¹ atoms/cm³ andAl_(y)Ga_(1-y)N single crystal layers (0≦y≦0.5) containing carbon from1×10¹⁷ atoms/cm³ to 1×10²¹ atoms/cm³ are alternately and repeatedlystacked in order, and a nitride active layer comprising an electrontransport layer having a carbon concentration of 5×10¹⁷ atoms/cm³ orless and an electron supply layer are deposited on a Si single crystalsubstrate in order, wherein the carbon concentrations of saidAl_(x)Ga_(1-x)N single crystal layers and the carbon concentration ofthe Al_(y)Ga_(1-y)N single crystal layers respectively decrease fromsaid substrate side towards said active layer side.
 2. The compoundsemiconductor substrate as claimed in claim 1, wherein saidAl_(y)Ga_(1-y)N single crystal layer is of 0.1≦y≦0.5.
 3. The compoundsemiconductor substrate as claimed in claim 1, wherein the carbonconcentration of said Al_(x)Ga_(1-x)N single crystal layer is higherthan the carbon concentration of Al_(y)Ga_(1-y)N single crystal layerimmediately above the former.
 4. The compound semiconductor substrate asclaimed in claim 2, wherein the carbon concentration of saidAl_(x)Ga_(1-x)N single crystal layer is higher than the carbonconcentration of Al_(y)Ga_(1-y)N single crystal layer immediately abovethe former.
 5. The compound semiconductor substrate as claimed in claim1, wherein said multilayer buffer layer contains boron from 5×10¹⁶atoms/cm³ to 1×10¹⁹ atoms/cm³.
 6. The compound semiconductor substrateas claimed in claim 2, wherein said multilayer buffer layer containsboron from 5×10¹⁶ atoms/cm³ to 1×10¹⁹ atoms/cm³.
 7. The compoundsemiconductor substrate as claimed in claim 3, wherein said multilayerbuffer layer contains boron from 5×10¹⁶ atoms/cm³ to 1×10¹⁹ atoms/cm³.8. The compound semiconductor substrate as claimed in claim 4, whereinsaid multilayer buffer layer contains boron from 5×10¹⁶ atoms/cm³ to1×10¹⁹ atoms/cm³.
 9. The compound semiconductor substrate as claimed inclaim 1, wherein said electron transport layer is an Al_(z)Ga_(1-z)Nsingle crystal layer (0≦z≦0.01).
 10. The compound semiconductorsubstrate as claimed in claim 2, wherein said electron transport layeris an Al_(z)Ga_(1-z)N single crystal layer (0≦z≦0.01).
 11. The compoundsemiconductor substrate as claimed in claim 3, wherein said electrontransport layer is an Al_(z)Ga_(1-z)N single crystal layer (0≦z≦0.01).12. The compound semiconductor substrate as claimed in claim 4, whereinsaid electron transport layer is an Al_(z)Ga_(1-z)N single crystal layer(0≦z≦0.01).
 13. The compound semiconductor substrate as claimed in claim5, wherein said electron transport layer is an Al_(z)Ga_(1-z)N singlecrystal layer (0≦z≦0.01).
 14. The compound semiconductor substrate asclaimed in claim 6, wherein said electron transport layer is anAl_(z)Ga_(1-z)N single crystal layer (0≦z≦0.01).
 15. The compoundsemiconductor substrate as claimed in claim 7, wherein said electrontransport layer is an Al_(z)Ga_(1-z)N single crystal layer (0≦z≦0.01).16. The compound semiconductor substrate as claimed in claim 8, whereinsaid electron transport layer is an Al_(z)Ga_(1-z)N single crystal layer(0≦z≦0.01).
 17. The compound semiconductor substrate as claimed in claim1, wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 18. The compound semiconductor substrate as claimed in claim 2,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 19. The compound semiconductor substrate as claimed in claim 3,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 20. The compound semiconductor substrate as claimed in claim 4,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 21. The compound semiconductor substrate as claimed in claim 5,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 22. The compound semiconductor substrate as claimed in claim 6,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 23. The compound semiconductor substrate as claimed in claim 7,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 24. The compound semiconductor substrate as claimed in claim 8,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 25. The compound semiconductor substrate as claimed in claim 9,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 26. The compound semiconductor substrate as claimed in claim 10,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 27. The compound semiconductor substrate as claimed in claim 11,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 28. The compound semiconductor substrate as claimed in claim 12,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 29. The compound semiconductor substrate as claimed in claim 13,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 30. The compound semiconductor substrate as claimed in claim 14,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 31. The compound semiconductor substrate as claimed in claim 15,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.
 32. The compound semiconductor substrate as claimed in claim 16,wherein said electron transport layer has a thickness of from 1 nm to500 nm and said substrate is used for a normally-off type switchingdevice.